Estimating the activation energy for hydrogen abstraction reactions by a combination of semiempirical calculation and family correlation

A semiempirical method, the PM3-family-correlation (PM3-FC) method, has bee n developed to estimate the activation energies for hydrogen abstraction re actions between hydrocarbon radicals and hydrogen donors. The method combin es semiempirical PM3 calculations of transition state enthalpies [DeltaH de grees (f)(TS)] with family correlations between the PM3-calculated and expe rimental DeltaH degrees (f)(TS) values on the basis of regression analysis. For a test set of 40 hydrogen abstraction reactions, including alkyl + alk yl-H, alkyl + allylalky-H-alpha/arylalkyl-H-alpha, and benzyl + allylalkyl- H-alpha/arylalkyl-H-alpha, it is found that the deviations between the PM3- calculated and experimental DeltaH degrees (f)(TS) values are systematic an d dependent on the transition state structures. The structurally homologous transition states show excellent linear correlation. Using the obtained li near regression parameters to scale the PM3-calculated DeltaH degrees (f)(T S) values leads to a very significant increase in estimation accuracy. The activation energies for the whole test set are estimated by using the scale d DeltaH degrees (f)(TS) values on the basis of the transition state theory . The average absolute deviation between the PM3-FC-estimated and experimen tal activation energies is 0.3 kcal/mol, with a standard deviation of 0.5 k cal/mol.